Determine the effectiveness of gas-filled microbubbles as contrast agents to be used in diagnostic ultrasound. Specifically: by means of sub-contract, procure appropriate gas-filled microbubbles of useful sizes; produce or procure in vitro systems to test the ultrasound contrast characteristics of the microbubbles; prepare appropriate tumor-bearing animals for in vivo testing of the contrast characteristics of the microbubbles; test the ultrasound contrast characteristics of the gas-filled microbubbles using a range of ultrasonic signals, with a variety of bubble sizes and compositions, using several bubble concentrations, rates of administration, sites of administration, and other combinations of conditions as required to provide the necessary information related to microbubbles as ultrasound contrast agents; and perform acute and chronic toxicity studies on promising microbubble preparations.